delihiros

(defun solver (x)
  (- (+ (sum-to 5 (- x 1) 0) (sum-to 3 (- x 1) 0)) (sum-to 15 (- x 1) 0)))

(defun sum-to (n x s)
  (cond ((eq x 0) s)
	((eq 0 (mod x n)) (sum-to n (- x 1) (+ s x)))
	(t (sum-to n (- x 1) s))))